In recent years, organic photoreceptors have been widely used in electrophotoreceptors. The organic photoreceptor is advantageous when compared to other photoreceptors because: the material for all types of exposure light sources, from visible light to infrared light, can be easily developed; materials which do not cause environmental pollution can be selected; and manufacturing cost is low. However, drawbacks of the organic photoreceptor are that mechanical strength and chemical durability are low, and when multiple sheet printing is done, there is deterioration of the electrostatic characteristics of the photoreceptor, or generation of defects on the photoreceptor surface and the like.
In other words, because electrical and mechanical external forces may be directly applied to the organic photoreceptor (hereinafter sometimes simply referred to as photoreceptor) by the charging means, developing means, transfer means and cleaning means, the organic photoreceptor must be resistant to these external forces.
More specifically, the photoreceptor must be resistant to wear or cracks generated on the photoreceptor surface due to friction; active oxygen such as ozone and the like that is generated when corona charging is done; and surface deterioration due to nitrogen oxides and the like.
In order to solve the above-described problems relating to mechanical and chemical durability, a structure is often employed in which the organic photoreceptor has a layer structure such that a charge generation layer and a charge transfer layer are superposed, and the charge transfer layer which is the protective layer is a uniform layer which is high in strength and through which an active gas cannot easily pass, and the thickness of the charge transfer layer is greater than 20 μm.
Also, there has been another approach of studying the technology of disposing a high-strength protective layer on the surface of the photoreceptor. For example, the use of a hard silicone resin as the protective layer of the photoreceptor has been reported (Patent Document 1). However, in the aforementioned methods in which the charge transfer layer is made thick or a high-strength protective layer is provided, there is the problem that the carrier generated at the charge transfer layer diffuses in the horizontal direction and reaches as far as the surface, and this causes problems with sharpness and the like. In the digital photocopier field, the demand for higher quality images has been increasing, and high resolution image formation is being researched, but in this type of layer structure or in the protective layer in which carrier diffusion occurs easily, a favorable electrostatic latent image cannot be obtained.
In order to obtain a high quality electrophotographic image, the image information must be reproduced as a latent electrostatic image so as to be faithful to the original, and the electric potential contrast of the exposed/unexposed parts on the organic photoreceptor must be formed so as to be sharp. However, in order to do this, it is necessary to limit the diffusion of the carrier generated so that it does not reach the surface charge. It has been reported that in latent image deterioration of a high density image, when the ratio D/μ of the diffusion constant of the charge transfer layer (D) and the drift mobility (μ) is large, the effect of diffusion to the electrostatic latent image cannot be disregarded, and when the charge transfer layer is thick, latent image deterioration increases (Non-patent Document 1).
In addition, it has been reported that a photoreceptor whose film is made thin has the effect of preventing diffusion of the toner image (Patent Document 2), and that high quality electrophotographic images are formed with a thin photoreceptor whose electrostatic capacity is large (Patent Document 3). Similarly, an organic photoreceptor has been proposed in which the charge transfer layer is made thin and diffusion of the electrostatic latent image is prevented (Patent Document 4).
However, when these thin organic photoreceptors which have been proposed are actually used in electrophotographic image forming apparatuses, when the image is formed, image unevenness due to uneven mechanical abrasion is more likely to be outstanding. Furthermore, it is more difficult to maintain image quality over a long period of time when compared with the organic photoreceptor of the prior art that has a thickness of 20 μm or more. It is thought that the reason for this is that as shown in FIG. 1, (which shows dependency of reduction in electric potential carrying capacity on layer thickness) in the case where the amounts of abrasion are the same, the reduction in electric potential carrying capacity (ΔV1>ΔV2) is larger as the thickness of the photosensitive layer gets smaller. Consequently, when a thin organic photoreceptor is used, image unevenness is likely to occur due to unevenness in abrasion of the organic photoreceptor.
In addition, when a thin organic photoreceptor is used and a developing agent having small particle diameter toner is employed in order to obtain high image quality, at those portions of the organic photoreceptor where abrasion unevenness occurs, mackle is likely to occur. This is thought to occur because of the presence on the organic photoreceptor, of toner components which tend to splash onto the rough regions which result from the unevenness of surface abrasion
[Patent Document 1]
Japanese Application Patent Laid-Open Publication No. Hei 06-118681 gazette
[Patent Document 2]
Japanese Application Patent Laid-Open Publication No. 2001-209206 gazette
[Patent Document 3]
Japanese Application Patent Laid-Open Publication No. 2001-312082 gazette
[Patent Document 4]
Japanese Application Patent Laid-Open Publication No. Hei 05-119503 gazette
[Non-Patent Document 1]
Japan Images Society Magazine, Volume 38 Issue 4, Page 296
The object of the present invention is to solve the aforementioned problems and thereby provide an electrophotographic image which uses a thin organic photoreceptor, and has high image quality over a long period, and to further provide an image forming method and image forming apparatus which use an organic photoreceptor in which there is little image unevenness or mackle.